Years of operating experience with nuclear reactors has revealed that nickel base alloy components in the primary (i.e., the reactor) coolant circuits such as steam generator tubes and tube plugs may be susceptible to primary water stress corrosion cracking in the reactor coolant. The susceptible components are fabricated from alloys which generally comprise at least about 60 weight percent nickel, about 13-30 weight percent chromium, and about 3-12 weight percent iron. Such nickel base alloys are generally members of the 600 and 700 alloy series, including Alloy 600, 690, X750 and the like. These alloys may also contain controlled amounts of cobalt, molybdenum, aluminum, titanium, columbium, tantalum, zirconium, copper, manganese, silicon and carbon. Inconel 600 alloy, for example, comprises at least 72 w/o nickel, 14-17 w/o chromium, 6-10 w/o iron, and no more than 0.5 w/o silicon, 1.0 w/o manganese, 0.15 w/o carbon, 0.5 w/o copper and 0.015 w/o sulfur. The symbol "w/o" is employed in this disclosure to indicate the element as "weight percent" of the total. The nuclear industry is very reluctant to add any element or compound to the primary coolant and thereby create unintended (and then unforeseeable) side effects. Any change on the primary coolant side is made only after conducting extensive qualification testing and the test results widely accepted by the nuclear industry.
In the last two decades researchers studying radiation causes in boiling water reactors have found that the addition of zinc to the primary coolant water advantageously reduces the buildup of radioactive deposits in the recirculating piping of the reactors. See, in this regard, Electric Power Research Institute (EPRI) Interim Report NP-4474, dated Mar. 1986, by W. J. Marble, entitled "BWR Radiation-Field Control Using Zinc Injection Passivation" and U.S. Pat. Nos. 4,759,900 and 4,756,874. (It has also been found in recent years that the addition of zinc to the primary coolant water of pressurized water nuclear reactors reduces the buildup of cobalt-60; see, in this regard, U.S. Ser. No. 184,557 filed Apr. 20, 1988 by the assignee of the present application.) The EPRI report states that small scale reactor qualification studies relating to the addition of zinc to the coolant fluid in an autoclave simulating boiling water nuclear reactors indicate that the addition of zinc does not appear to increase the susceptibility of sensitized stainless steel to intergranular stress corrosion cracking. This is a particular concern because stainless steel is a principal material of construction employed in boiling water nuclear reactors. In addition, Section 6 of the EPRI report further indicates that the researchers also determined that addition of zinc to the autoclave did not degrade the intergranular stress corrosion resistance of Inconel 600 and X750 test coupons in 290.degree. C. water. These coupons were removed from the autoclave and then tested using constant extension rate and creviced bent beam procedures.